Given that it doesn't have and area for droop screws on the chassis, I would say its a RS4 pro.
chensleyrc1 is correct that the original RS4 was 3 gear, they made the change with the pro version.
Here'a a link to see the manuals (Rs4, RS4 pro and the Pro 2
http://www.oldrc.com/OLDRC/HPI_Manuals.html